Monday 24 June 2019

Isisfordia molnari: A new species of Crocodyliform from the Middle Cretaceous of New South Wales.

The Crocodyliforms were the only group of Crocodylomorphs to survive the End Triassic Extinction event. The group includes all modern Crocodylians, but also a number of extinct groups, such as the Dog-like Peirosaurians, the short-snouted, long-legged Notosuchins, the fully marine Metriorhynchids, and the Dyrosaurids, a diverse group which survived till the Eocene. These Crocodyliforms were abundant in the Mesozoic of most of the Southern Hemisphere, but are startlingly absent from the Australian continent, where only a single species has been described, Isisfordia duncani, from the Middle Cretaceous of Queensland, a species thought to be closely related to, but not placed within, the still extant Crocodylians.

In a paper published in the journal PeerJ on 21 June 2019, Lachlan Hart and Phil Bell of the Palaeoscience Research Centre at the University of New England, Elizabeth Smith of the Australian Opal Centre, and Steven Salisbury of the School of Biological Sciences at the University of Queensland describe a second species of Isisfordia from two opalised bone fragments from mines in the Lightning Ridge area of New South Wales and Queensland.

Opal is an amorphous form of silica containing as much as 21% water, It is made up of tiny spheroids of crystalline silica some 150 to 300 nm in diameter, with random alignments to one another. Light passing through these crystaloids is refracted as by a prism, leading to the 'opalescent' sheen which gives the mineral its value. Opal is formed by water percolating through silica rocks, where it dissolves some of the mineral forming a silica-rich solution. This can accumulate in any cracks or gaps in the rock, where if it reaches a high enough concentration it can precipitate out as opal. The Cretaceous Griman Creek Formation of northern New South Wales and southern Queensland is noted for the occurrence of opalised vertebrate fossils, replacement fossils formed when opal forms in gaps in the rock left by the dissolution of bone or earlier replacement material. However, these are rare and are not always spotted in areas where the opal is mined using mechanised equipment, which also means that the material that is recovered is often fragmented.

The new species is named Isisfordia molnari, in honour of Ralph Molnar, for his work on Crocodylomorphs from the Griman Creek Formation, and Australian vertebrate palaeontology in general. The species is described from two specimens, a braincase recovered from a mine in 2000 and donated to the Australian Museum by opal miners Peter and Lisa Caroll, and a partial maxilla (upper jawbone) extracted in 1914 and donated to the Australian Museum by Colonel Reuter Emerich Roth. The precise locations where these specimens were found is unclear, though the jaw was probably from Three Mile Field, close to the town of Lightning Ridge, and the braincase is probably from Coocoran Fields, 30 -40 km to the west.

Isisfordia molnari, maxillary fragment from the Griman Creek Formation. (A) medial, (B) lateral, (C) palatal views. Arrows indicate rostral end. Scale bars equal 10 mm. Abbreviations: ag, alveolar groove; is, interalveolar septa; ms, medial `shelf'; pp, palatal process. Hart in Hart et al. (2019).

Isisfordia molnari is differentiated from Isisfordia duncani on the basis of the shape of the pariedal bones (the bones which form the side and roof of the skull in Humans), which are flat in Isisfordia molnari and have ridges on them in Isisfordia duncani, and the shape and spacing of the tooth sockets, which are rounded in Isisfordia duncani  and oval in Isisfordia molnari.

Reconstruction of Isisfordia molnari in life, swimming next to a wading Sauropod. José Vitor Silva in Hart et al. (2019).

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